GIMP#15 Fading Frames: Transparent frames using the Blend tool (Gimp 2.8)
There are many ways to create frames and borders in Gimp 2.8, but this shows a quick and easy approach to creating transparent frames with zing, even if your border is irregular and not a rectangle. The key points to remember are:
1) Create a transparent layer the same size as your original image (which it will be by default when you use New layer).
2) Switch to Overlay for the transparent layer, though you may later either switch to Hard Light or Grain Merge, or alternatively change the opacity of the transparent layer which contains the frame.
3) Use the Blend tool, which I also call the gradient tool, and set the Shape to Shaped (spherical). This creates a shape that hugs the border of the selection, which is the entire layer if nothing is selected.
4) Try out various gradients, especially those with some transparency such as Square Wooden Frame, Tube Red, Radial Rainbow Hoop and others. After each gradient, be sure to clear the layer before trying another, as some will mix together (although that can be interesting as well).
5) Don't forget that after creating the frame you like, you can alter the Hue, the Brightness/Contrast and more to get just the look you want.